Ion generation is used in a wide variety of applications including, for example, ion implantation, thin film formation, etching and sputtering operations, propulsion in space ships, electrostatographic devices, electro-static air cleaners, for the generation of negative ions for medicinal purposes and electro-hydrodynamic gas pumps.
As is understood in the field of ion generation, a gas such as air can be thought of as a non-linear circuit element. At temperatures below 1000° C. and electric fields below the dielectric breakdown point, gasses are insulating and free of ions (M. Boulos, P. Fauchais and E. Pfender, Thermal Plasmas—Fundamentals and Applications, Plenum Press, New York, 1994). Under these conditions, no current will flow between a pair of electrodes separated by a gas gap. However, there is a point where the potential difference between two electrodes can be high enough to cause the gas to breakdown and transition from an electrical insulator to a conductor.
FIG. 1 illustrates one method of ion generation. The process begins with a small number of seed electrons that are free to pass through the gas. These free electrons can be released from the gas by random photo-ionization or collision-ionization. The electrons can also be emitted from the cathode electrode by processes collectively referred to as secondary electron emission (L. Loeb, Fundamental Processes of Electrical Discharge in Gases, Wiley & Sons, London, 1947). These cathode processes include ion bombardment, photo-ionization and other processes. In the presence of a high electric field, the free electrons interact with the neutral gas molecules to create ions and additional electrons. An avalanching chain reaction takes place and the number of electrons and ions in the gas increases rapidly over time (shown in the drawing as proceeding left to right). As a result of this chain reaction, very quickly (0.05 to 10 μs), the gas evolves into a high-temperature plasma or glow, which is a state where a significant number of molecules are ionized and a large amount of free electrons are present (E. Nasser, Fundamentals Of Gaseous Ionization And Plasma Electronics, Wiley-Interscience, New York, 1971). In this state the gas is at a high temperature and is highly conductive. Gas in this state is fundamentally different from an insulating state.
The various conventional approaches to ion generation according to the prior art can be gleaned from certain issued U.S. patents
For example, in U.S. Pat. No. 6,373,680, entitled “Method and device for ion generation,” a time varying corona discharge is used to generate ions. The time varying corona discharge is created in air by relatively slow voltage pulses between corona electrodes. During the pulse, a corona discharge is established. The pulse duration is short enough such that ions generated in the corona do not have time to reach a neutralizing electrode before the pulse is turned off. With the electric field turned off, the ions are exhausted to the ambient air by a fan. Although the voltage is pulsed, the frequency is low and the corona discharge is fully developed during each pulse. The field is turned off after the gas region fills with ions. The field is removed mainly to aid in the ejection of ions into the ambient air.
Another example is U.S. Pat. No. 5,841,235 entitled “Source for the generation of large area pulsed ion and electron beams.” This patent describes a vacuum arc plasma source where the discharge current is controlled by a parallel circuit including an ohmic resistor and a capacitor. The difference with this prior art is that the discharge created is a plasma type discharge. Ions are not created at ambient temperature, but at a high temperature.
Several other patents relate to various aspects of ion generation using a corona discharge. Generally, these approaches generate ions under steady-state or near steady-state conditions. This group of patents relies on the partial breakdown of air that is found with a sharp-blunt electrode pair at high voltages. These patents include U.S. Pat. No. 6,703,785 entitled “Negative ion generator,” U.S. Pat. No. 5,977,716 entitled “Ion generator for a combustion device,” U.S. Pat. No. 4,559,467 entitled “Ion-generator for producing an air flow,” U.S. Pat. No. 6,061,074 entitled “Ion generator for ionographic print heads,” U.S. Pat. No. 5,973,905 entitled “Negative air ion generator with selectable frequencies,” U.S. Pat. No. 4,185,316 entitled “Apparatus for the generation of ions,” U.S. Pat. No. 4,038,583 entitled “Apparatus for the generation of negative or positive atmospheric ions.”
U.S. Patent Publication No. 2005/0007726 A1 describes a unique ion generating process. This invention uses electrons emitted from a nano-featured cathode by a quantum tunneling process. The electrons are then reacted with the gas to create unipolar ions without inducing an avalanche. The avalanche is avoided by placing electrodes only a few microns apart —too short of a distance to develop the chain reaction shown in FIG. 1. The disadvantages of this approach include a sensitivity of the nano-featured cathode emitter to contamination and damage, and an inability to produce a large enough number of ions.
Another example of the shortcomings of the prior art is U.S. Pat. No. 5,434,469 entitled “Ion generator with ionization chamber constructed from or coated with material with a high coefficient of secondary emission.” This ion generator is for ion beam equipment and attempts to improve ion generation efficiency by increasing the secondary electron emission coefficient of the chamber walls in a plasma ion generator. This prior art describes a second alternative for ion generation where high temperature ions are formed in a high temperature plasma.
These and other prior art approaches have many shortcomings. Individually and collectively, these include requiring high voltage, high amounts of input energy and operating at high temperatures and pressures for ion generation. Electrodes are exposed to the hostile environment of a high-temperature plasma and suffer from degradation effects. Ions generated by the prior art are not suited for various applications such as cooling systems because, for example, introducing ions at a high temperature would limit or eliminate the heat removal ability of such a system.
Accordingly, there exists a need in the art for an improved ion generation technique over corona discharge that is well suited for various applications such as cooling systems, along with the ability to operate in ambient conditions of temperature and pressure, and without the need for special electrodes.